The present invention relates to the art of blowmolding free standing plastic drums, and more specifically to a bunghole structure for blowmolded plastic drums. The steel drum has been the most successful drum in the recent past utilized for purposes of handling, storing, and transporting bulk quantities of fluid materials on the order of at least 30 gallons, and more typically on the order of about 50 to 60 gallons. The drum most commonly employed has a nominal volume of approximately 55 gallons.
More recently, however, freestanding tight-head plastic drums have been developed and made available, but they have not yet obtained their full potential. It should be appreciated that such drums, especially those which are molded as a single piece article, offer many highly desirable characteristics. Some of these desirable characteristics include, for example, low price, low shipping costs, a wide scope of product applications without the need for liners, high resistance to the detrimental influence of weather, lightweight, rust resistance, dent resistance, and aesthetic appearance. Some of the deterrents to the wider acceptance and usage of plastic drums have been cross-threading of the plastic threads and warping of the sealing surface in the bunghole.
Cross-threading occurs most frequently when the user has difficulty screwing the closures and other devices into the bunghole. The user oftentimes forces the engagement without realizing that, because of the relatively soft plastic, the bunghole threads may be permanently damaged, thereby rendering the drum useless for further intended needs. The difficulty of insertion may be due to a canted closure or to warping of the bunghole itself. Additionally, however, cross-threading or other damage to the plastic threads also occurs during government required quality control drop tests such as under the United States Department of Transportation Regulations Governing Transportation of Hazardous Materials Section 178.19, Specification 34. The deformation of the bunghole and closures occurs at the moment of impact resulting in either immediate expulsion of the closure or, as is the case with all cross-threaded or otherwise damaged bungholes, in leakage or contamination of the contents.
Warping of the bunghole, and especially the sealing surface, results from uneven drum wall shrinkage of the plastic during the post-blow mold cooling stage of manufacture. Such warping prevents the formation of a good seal without the use of sufficiently thick and elastic gaskets. One solution to the warping problem has been the technique of insert blowmolding over a prefabricated threaded plastic flange. However, the technique did nothing for the cross-threading problem and sometimes, created leakage difficulties at the body/flange interface.
Attempts have been made to insert blowmold over threaded metal flanges but have met with failure, primarily due to leakage between the plastic-to-metal interface. The leakage problem can be better appreciated by an understanding of some of the other tests which the drums commonly must pass. Typical government regulations require plastic drums to retain a hydrostatic internal pressure of 15 PSI at equilbrium for five minutes without showing a pressure drop or evidence of leakage. Such pressure is often adequate to break the minimal seal at the plastic-to-metal interface, thereby resulting in a failure of the test. Another test requires a 30 gallon plastic drum filled to 98 percent capacity with water to withstand a compressive load of 1800 pounds applied to the load bearing areas of the top of the container for a period of not less than 48 hours without buckling of the side walls sufficient to cause damage, and in no case may the maximum top to bottom deflection be more than one inch. This test generally applies only to 30 gallon and smaller drums. The 55 gallon drum must pass all of the tests for the smaller drums, except that the load to be applied in the compression test is 2400 pounds. Until the present invention, almost all plastic drums constructed with insert blowmolded metal flanges failed the compression test because they leaked at the plastic -to- metal interface. Even leakage of the headspace air in the drum resulted in buckling and excessive top to bottom deflection since the entrapped air provides a necessary resistance to the above-described high compressive forces. Attempts have been made to solve the leakage problem by adhering the plastic to the metal insert but have not been successful.